1. Field of the Invention
The invention relates generally to concrete additives, and more particularly, to methods for reducing the evolution of ammonia from fly ash contaminated with ammonia in cementitious and pozzolanic mixtures.
2. Background Information
Fly ash is produced from burning pulverized coal in a coal-fired boiler. It is a finegrained, powdery particulate material that is carried off in the flue gas and may be collected from the gas by electrostatic precipitators or mechanical collection devices, including cyclones. Fly ash is typically disposed in landfill areas.
Fly ash may be used in concrete as a partial replacement for Portland cement, and also as a filler. The presence of fly ash in cement mixtures or concrete has several advantages, including, but not limited to, the reduction of fly ash disposed in landfills, a lower cost than other additives, and a decrease in permeability and shrinkage in the hardened concrete. Other advantages include improved workabililty, durability and long-term strength gain, due to reduced dry shrinkage, reduced heat of hydration, water reduction, reduced alkali silica reactivity, and increased resistance to sulfate attack. Where workability is improved, there is reduced bleeding and segregation, improved flowability, and improved finishing characteristics. In addition, there is the reduction of the greenhouse gas, carbon dioxide, released into the atmosphere by the replacement of a portion of the cement with fly ash.
The use of fly ash in concrete, however, is not without drawbacks. The chemical and physical properties of fly ash are influenced by the coal burned and the handling techniques. The properties are also influenced by the individual combustion techniques and associated efficiency of the individual boilers. Nitrogen compounds, including NO and NO2, referred to generically as NOx, are generated through the combustion of coal. Several technologies have been developed to meet the mandated NOx reduction limits of the Clean Air Act Amendments (1990) of the United States Environmental Protection Agency. The technologies include selective non-catalytic reduction, selective catalytic reduction, and electrostatic precipitation systems. The processes include the injection of ammonia into the flue gas of the power plant to improve performance of the particular process. As a result, some of the ammonia remains unreacted, and is deposited on the fly ash, primarily in the form of ammonium sulfate.
Thus, fly ash that has been treated with ammonia to reduce NOx may be undesirable for use in cementitious or pozzolanic mixtures. For example, when ammonium-containing fly ash is added to an highly alkaline aqueous concrete mixture, the ammonium cations are converted to ammonia gas, which evolves from the cementitious slurry into the air, thus exposing workers to potentially hazardous conditions. The OSHA (Occupational, Safety and Health Administration) threshold and permissible limits are set at 25 ppm for a time-weighted average of eight hours, and 35 ppm for a short term exposure limit of 15 minutes.
A need remains for a method for reducing the evolution of ammonia or other ammonium compounds from contaminated fly ash in cementitious and pozzolanic compositions.